Reviewed by Michael Natterer.

* gtk/gtkstyle.c,
* gtk/gtkstyle.h: gdk_draw_string() has been deprecated and removed
  already. There's no one left who should be using this function
  (found this when porting sapwood to GTK+ 3.0)
* modules/engines/pixbuf/pixbuf-draw.c: don't implement this unused
  function